The blood coagulation pathway, in part, involves the formation of an enzymatic complex of Factor Villa (FVIIIa) and Factor IXa (FIXa) (Xase complex) on the surface of platelets. FIXa is a serine protease with relatively weak catalytic activity without its cofactor FVIIIa. The Xase complex cleaves Factor X (FX) into Factor Xa (FXa), which in turn interacts with Factor Va (FVa) to cleave prothrombin and generate thrombin. Hemophilia A is a bleeding disorder caused by mutations and/or deletions in the FVIII (FVIII) gene resulting in a deficiency of FVIII activity (Peyvandi et al. 2006). In some cases, patients have reduced levels of FVIII due to the presence of FVIII inhibitors, such as anti-FVIII antibodies.
Hemophilia A is characterized by spontaneous hemorrhage and excessive bleeding. Over time, the repeated bleeding into muscles and joints, which often begins in early childhood, results in hemophilic arthropathy and irreversible joint damage. This damage is progressive and can lead to severely limited mobility of joints, muscle atrophy and chronic pain (Rodriguez-Merchan, E. C., Semin. Thromb. Hemost. 29:87-96 (2003), which is herein incorporated by reference in its entirety).
The disease can be treated by replacement therapy targeting restoration of FVIII activity to 1 to 5% of normal levels to prevent spontaneous bleeding (see, e.g., Mannucci, P. M., et al., N. Engl. J. Med. 344:1773-9 (2001), herein incorporated by reference in its entirety). There are plasma-derived and recombinant FVIII products available to treat bleeding episodes on-demand or to prevent bleeding episodes from occurring by treating prophylactically. Based on the half-life of these products (10-12 hr) (White G. C., et al., Thromb. Haemost. 77:660-7 (1997); Morfini, M., Haemophilia 9 (suppl 1):94-99; discussion 100 (2003)), treatment regimens require frequent intravenous administration, commonly two to three times weekly for prophylaxis and one to three times daily for on-demand treatment (Manco-Johnson, M. J., et al., N. Engl. J. Med. 357:535-544 (2007)), each of which is incorporated herein by reference in its entirety. Such frequent administration is inconvenient and costly.
A major impediment in providing a low-cost recombinant FVIII protein to patients is the high cost of commercial production. FVIII protein expresses poorly in heterologous expression systems, two to three orders of magnitude lower than similarly sized proteins. (Lynch et al., Hum. Gene. Ther.; 4:259-72 (1993). The poor expression of FVIII is due in part to the presence of cis-acting elements in the FVIII coding sequence that inhibit FVIII expression, such as transcriptional silencer elements (Hoeben et al., Blood 85:2447-2454 (1995)), matrix attachment-like sequences (MARs) (Fallux et al., Mol. Cell. Biol. 16:4264-4272 (1996)), and transcriptional elongation inhibitory elements (Koeberl et al., Hum. Gene. Ther.; 6:469-479 (1995)).
Advances in our understanding of the biology of FVIII expression has led to the development of more potent FVIII variants. For instance, biochemical studies demonstrated that the FVIII B-domain was dispensable for FVIII cofactor activity. Deletion of the B-domain resulted in a 17-fold increase in mRNA levels over full-length wild-type FVIII and a 30% increase in secreted protein. (Toole et al., Proc Natl Acad Sci USA 83:5939-42 (1986)). This led to the development of B domain-deleted (BDD) FVIII protein concentrate, which is now widely used in the clinic. Recent studies, however, indicate that full length and BDD hFVIII misfold in the ER lumen, resulting in activation of the unfolded protein response (UPR) and apoptosis of murine hepatocytes.
Thus, there exists a need in the art for FVIII sequences that express efficiently in heterologous systems.